Prostate cancer is currently the most common type of cancer in American men and the second leading cause of cancer related death in this population. In its advanced stages, prostate cancer metastasizes preferentially to bone, where it forms osteoblastic lesions. After initial treatment with androgen ablation therapy, most metastatic prostate cancers become hormone-refractory and lethal. Current diagnostic and therapeutic modalities are limited by a lack of specificity and an inability to predict which patients are at risk of developing metastatic disease.
Most prostate cancers initially occur in the peripheral zone of the prostate gland, away from the urethra. Tumors within this zone may not produce any symptoms and, as a result, most men with early-stage prostate cancer will not present clinical symptoms of the disease until significant progression has occurred. Tumor progression into the transition zone of the prostate may lead to urethral obstruction, thus producing the first symptoms of the disease. However, these clinical symptoms are indistinguishable from the common non-malignant condition of benign prostatic hyperplasia (BPH).
One of the fundamental problems in the diagnosis and treatment of prostate cancer is the lack of a marker that can accurately detect early-stage, localized tumors. Although a number of markers have been identified and some, like PSA, are in widespread clinical use, the ideal prostate tumor marker has yet to be characterized. A similar problem is the lack of an effective prognostic marker for determining which cancers are indolent and which ones are or will be aggressive. PSA, for example, fails to discriminate accurately between indolent and aggressive cancers. In addition, there is also a great need for markers that might serve as targets for therapeutic methods such as antibody-directed therapy, immunotherapy, and gene therapy.
Currently, there is no effective treatment for the 20-40% of patients who develop recurrent disease after surgery or radiation therapy or for those patients who have metastatic disease at the time of diagnosis. Although hormone ablation therapy can palliate these patients, the majority inevitably progresses to develop incurable, androgen-independent disease (Lalani et al., 1997, Cancer Metastasis Rev. 16: 29-66).
Early detection and diagnosis of prostate cancer currently relies on digital rectal examinations (DRE), prostate specific antigen (PSA) measurements, transrectal ultrasonography (TRUS), and transrectal needle biopsy (TRNB). At present, serum PSA measurement in combination with DRE represents the leading tool used to detect and diagnose prostate cancer. Both have major limitations that have fueled intensive research into finding better diagnostic markers of this disease.
PSA is the most widely used tumor marker for screening, diagnosing, and monitoring prostate cancer today. In particular, several immunoassays for the detection of serum PSA are in widespread clinical use. Recently, a reverse transcriptase-polymerase chain reaction (RT-PCR) assay for PSA mRNA in serum has been developed. However, PSA is not a disease-specific marker, as elevated levels of PSA are detectable in a large percentage of patients with BPH and prostatitis (25-86%) (Gao et al., 1997, Prostate 31: 264-281), as well as in other nonmalignant disorders and in some normal men, a factor which significantly limits the diagnostic specificity of this marker. For example, elevations in serum PSA of between 4 to 10 ng/ml are observed in BPH, and even higher values are observed in prostatitis, particularly acute prostatitis. BPH is an extremely common condition in men. Further confusing the situation is the fact that serum PSA elevations may be observed without any indication of disease from DRE, and vice-versa. Moreover, it is now recognized that PSA is not prostate-specific (Gao et al., supra, for review).
Various methods designed to improve the specificity of PSA-based detection have been described, such as measuring PSA density and the ratio of free vs. complexed PSA. However, none of these methodologies have been able to reproducibly distinguish benign from malignant prostate disease. In addition, PSA diagnostics have sensitivities of between 57-79% (Cupp & Osterling, 1993, Mayo Clin Proc 68:297-306), and thus miss identifying prostate cancer in a significant population of men with the disease.
Prostate-Specific Membrane Antigen (PSMA) is a recently described cell surface marker of prostate cancer that has been the subject of various studies evaluating its use as a diagnostic and therapeutic marker. PSMA expression is largely restricted to prostate tissues, but detectable levels of PSMA mRNA have been observed in brain, salivary gland, small intestine, and renal cell carcinoma (Israeli et al., 1993, Cancer Res 53: 227-230). PSMA protein is highly expressed in most primary and metastatic prostate cancers, but is also expressed in most normal intraepithelial neoplasia specimens (Gao et al., supra). Preliminary results using an Indium-111 labeled, anti-PSMA monoclonal antibody (mAb) to image recurrent prostate cancer show some promise (Sodee et al., 1996, Clin Nuc Med 21: 759-766). Whether PSMA will prove to be a useful therapeutic target remains to be determined. However, PSMA is a hormone dependent antigen requiring the presence of functional androgen receptor. Since not all prostate cancer cells express androgen receptor, PSMA's utility as a therapeutic target may be inherently limited.
Clinical staging of prostate cancer is another fundamental problem facing urologists today. Currently, clinical staging relies on rectal examination to determine whether the tumor remains within the borders of the prostatic capsule (locally confined) or extends beyond it (locally advanced), in combination with serum PSA determinations and transrectal ultrasound guided biopsies. However, because of the subjectivity involved, clinical staging by DRE regularly underestimates or overestimates local extension of the tumor, frequently misjudges its location, and correlates poorly with volume and extent of the tumor (Lee, C. T. and Osterling, J. E. Cancer of the Prostate: Diagnosis and Staging. In: Urologic Oncology, W. B. Saunders Company, Philadelphia, pp 357-377 (1997)). Serum PSA levels are also utilized for staging purposes, but PSA alone has not been able to reliably stage prostate tumors. No technique has proven reliable for predicting progression of the disease. Thus, there is a need for more reliable and informative staging and prognostic methods in the management of prostate cancer.